Patent Document 1 discloses a vehicle anti-theft apparatus configured to prevent theft of a parked vehicle. The vehicle anti-theft apparatus determines the situation around the vehicle based on the output from a vehicle exterior camera, a radar, and a proximity sensor, and issues a warning to a suspicious person by turning on an indoor light, or ringing a horn, for example.
On the other hand, when a user sits in a parked vehicle, a suspicious person may suddenly enter the vehicle from its unlocked door, and the user may be victim to a crime. In order to monitor suspicious persons, it is conceivable to utilize the vehicle anti-theft apparatus disclosed in Patent Document 1.